Refrigerant Charge Storage

ABSTRACT

A refrigeration system includes a compressor, first and second heat exchangers, and an expansion device. A refrigerant recirculating flowpath extends sequentially downstream through the compressor, first heat exchanger, expansion device, and second heat exchanger The system includes a charge storage system. The charge storage system includes first and second refrigerant storage chambers. At least one valve is coupled to the storage chambers to permit the storage chambers to each be individually placed in alternative communication with the flowpath upstream and downstream of the expansion device.

BACKGROUND OF THE INVENTION

The invention relates to refrigeration. More particularly, the invention relates to transcritical refrigeration systems used for transport or commercial refrigeration.

As a natural and environmentally benign refrigerant, CO₂ (R-744) is attracting significant attention. The critical temperature for CO₂ is 87.8° F. In most air-conditioning and refrigerating operating conditions, the heat rejection occurs above this temperature so that CO₂ systems operate in transcritical mode.

Different applications will require different ranges of operation (e.g., ranges of gas cooler and evaporator conditions). For example, a beverage cooler may have an essentially fixed desired interior condition (e.g., very close to 34-38° F., to avoid risk of frezing, but still provide cooling). This temperature essentially fixes the steady state compressor suction pressure. It is unlikely any operator would seek to run a beverage cooler at a different temperature. Other applications, such as transport refrigeration units (e.g., truck boxes, trailers, cargo containers, and the like), require broader capabilities. A given unit configuration may be made manufactured for multiple operators with different needs. Many operators will have the need to, at different times, use a given unit for transport of frozen goods and non-frozen perishables. An exemplary frozen goods temperature is about −10° F. or less and an exemplary non-frozen perishable temperature is 34-38° F. The operator will predetermine appropriate temperature for each of the two modes. Prior to a trip or series, the technician or driver will enter the appropriate one of the two temperatures. Other operators may have broader requirements (e.g., an exemplary overall range of −40-57° F.).

Typically with variation in operating conditions, the mass flow rates and densities of the refrigerant vary greatly. For a system with fixed amount of active (circulating) charge this might cause uneven refrigerant pressure and temperature control and interfere with system performance. Additionally, the sensitivity of CO₂ to operating conditions, the relatively high pressures of operation, and the lack of two-phase state at typical charge storage points, can cause more problems. Accordingly, various charge storage systems have been proposed to permit selective withdrawal of refrigerant from circulation to allow the system to be operated more advantageously. Besides operational issues, the storage vessel, if isolated from the system, could be exposed to very high ambient temperatures. If loaded with charge, the high ambient temperatures may cause significant pressure increases. The pressure increases could cause vessel rupture.

U.S. Pat. No. 7,096,679 discloses heating/cooling a reservoir to modulate the amount of refrigerant returned. Heating increases the heat load on the system, thereby making the system less efficient. The heating and cooling may increase the power consumption in the system. U.S. Pat. No. 6,385,980 discloses a flash tank economizer. If the flash tank economizer vapor line is closed for some operating conditions, then the pressure inside the flash tank may increase as described above. Other systems include an accumulator at the downstream end of the evaporator as a charge storage device. These may suffer from excessive oil build up in the bottom of the accumulator and liquid sloshing into the compressor at system startup.

Thus, this present disclosure may address one to all the above problems, and provide means for regulating charge in the system over same to the entire operating envelope of typical transport and commercial applications.

SUMMARY OF THE INVENTION

Accordingly, one aspect of the invention involves a refrigeration system including a compressor, first and second heat exchangers, and an expansion device. A refrigerant recirculating flowpath extends sequentially downstream through the compressor, first heat exchanger, expansion device, and second heat exchanger. The system includes a charge storage system. The charge storage system includes first and second refrigerant storage chambers. At least one valve is coupled to the storage chambers to permit the storage chambers to each be individually placed in alternative communication with the flowpath upstream and downstream of the expansion device.

The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially schematic view of a first refrigeration system.

FIG. 2 is a partially schematic view of a second refrigeration system.

FIG. 3 is a view of a refrigerated transport unit.

Like reference numbers and designations in the various drawings indicate like elements.

DETAILED DESCRIPTION

FIG. 1 schematically shows a transcritical vapor compression system 20 utilizing CO₂ as working fluid (refrigerant). The system comprises a compressor 22 (e.g., a reciprocating, a scroll, or screw compressor having an electric motor), a heat rejection heat exchanger (gas cooler) 24, an expansion device 26, and a heat absorption heat exchanger (evaporator) 28 in sequential order along a recirculating primary flowpath. The exemplary gas cooler and evaporator may each take the form of a refrigerant-to-air heat exchanger.

Airflows across one or both of these heat exchangers may be forced. For example, one or more fans 30 and 32 may drive respective airflows 34 and 36 across the two heat exchangers. The conduits along the primary refrigerant flowpath 40 include a suction line 42 extending from an outlet 44 of the evaporator 28 to an inlet 46 of the compressor 22. A discharge line 48 extends from an outlet 50 of the compressor to an inlet 52 of the gas cooler. Additional lines 54 and 56 respectively connect the gas cooler outlet 58 to expansion device inlet 60 and expansion device outlet 62 to evaporator inlet 64.

An exemplary expansion device 26 is an electronic expansion valve (commonly identified as an EEV or EXV). An electronic expansion valve typically comprises a stepper motor attached to a needle valve to vary the effective valve opening or flow capacity. The opening of the valve may be electronically controlled by a controller 66 which may also control operation of the compressor and other system components. The controller may operate in response to input from one or more user input devices 68 (e.g., switches, electronic controls, and the like) and one or more sensors (e.g., evaporator outlet temperature and/or pressure, discharge pressure and/or temperature, ambient and controlled space temperatures).

For a desired operating condition of the system, and depending on the performance of individual components of the system, there will be a particular discharge pressure at which the system operates at maximum efficiency and there will be a particular discharge pressure where the system operates at maximum capacity. While the system is going through a pulldown process, it might be advantageous that the system follow the discharge pressure which provides maximum capacity. When a steady state is reached, it might be advantageous that the system follow the discharge pressure which provides optimal efficiency (or be somewhere in between the two pressures to be optimized for a combination of efficiency and capacity). Both for operating the cycle at a given condition and for maintaining the system at the desired discharge pressure for that condition, there will be an associated optimal amount of refrigerant circulating along the flowpath 40. Because the total system charge is fixed, a charge storage system 80 is used to store refrigerant from flowpath 40 and return refrigerant to the flowpath 40 so that the circulating charge will more closely correspond to the optimal charge as may be appropriate to maintain desired system performance.

In general, as the evaporator temperature goes down, the liquid refrigerant density in the evaporator increases and greater mass of refrigerant gets stored in the evaporator. In the absence of intervention, the mass flow rate of the circulating charge decreases. At that condition it is desirable to store the least amount of refrigerant in the system 80. Similarly, when the heat exchangers are at their highest temperatures, the evaporator will store a relatively low amount of refrigerant. To avoid overpressurizing the system 20, it is desirable to store the most refrigerant in the storage system 80. Thus, during system startup and pulldown it is desirable to have a maximum amount of charge in the storage system 80. As the evaporator temperature goes down, the storage system 80 may be controlled to unload progressively more charge into the active cycle.

The exemplary system includes a plurality of reservoirs 82, 83, and 84 whose chambers 85, 86, and 87 are fluidically coupled in parallel with each other and with the expansion device. The reservoirs may each be opened and closed to the primary flowpath 40 by valves at high and low pressure ends of the reservoirs. For purposes of illustration, each reservoir is shown having an associated first (high pressure) valve 90, 91, and 92 between that reservoir's inlet 93, 94, and 95 and the expansion device inlet location/condition 60. Each reservoir further has an associated second (low pressure) valve 96, 97, and 98 between a second port 99, 100, and 101 of that reservoir and the expansion device outlet location/condition 62. As is discussed further below, various of the first valves may be integrated with each other, first and second valves may be integrated with each other, or other combinations (e.g., using four-way or greater valve structures).

In an exemplary method of operation, opening and closing of the first and second valves is controlled by the controller responsive to a combination of measured/sensed conditions and/or user-entered parameters (e.g., set temperatures). In the exemplary method, under normal operating conditions, each reservoir has exactly one of its two valves open while the other valve is closed. The selection of the appropriate combination of open and closed valves will determine the effective charge storage of the system 80.

For each reservoir, the amount of charge stored in the reservoir will be determined by system conditions at whichever of its first and second valves (or associated ports) is open. If the first valve is open, the reservoir will be exposed to the relatively high pressure expansion device inlet conditions. The reservoir will, therefore, hold a relatively high charge amount. If, however, the second valve is open, the reservoir will be exposed to relatively low pressure suction conditions and a relatively small amount of charge will be stored.

Thus, a condition of maximum stored charge and minimum circulating charge is associated with all of the first valves being open and all of the second valves being closed. Likewise, a condition of minimum stored charge and maximum circulating charge is associated with all of the first valves being closed and all of the second valves being open. Other combinations of closed and open valves provide one or more intermediate conditions. The nature of those intermediate conditions will depend upon the relative and absolute sizes of the reservoirs.

In an exemplary reservoir sizing, the relative sizes of the first and second reservoirs are selected so that the effective capacity of the second reservoir is twice that of the first reservoir (i.e., the difference in charge amount held by the second reservoir between its two conditions is twice that of the first). Similarly, the third reservoir is selected to have an effective capacity twice that of the second. The absolute sizes of the reservoirs are selected so that the combined effective capacities provide a desired overall charge storage/buffering capacity. With this exemplary combination of reservoir sizes, six evenly separated intermediate conditions may be obtained between the minimum stored charge and maximum stored charge conditions.

FIG. 2 shows a more basic system with just the first and second reservoirs so that a total of four charge storage conditions can be achieved.

FIG. 3 shows a refrigerated transport unit (system) 220 in the form of a refrigerated trailer. The trailer may be pulled by a tractor 222. The exemplary trailer includes a container/box 224 defining an interior/compartment 226. An equipment housing 228 mounted to a front of the box 224 may contain an electric generator system including an engine 230 (e.g., diesel) and an electric generator 232 mechanically coupled to the engine to be driven thereby. The refrigeration system 20 may be electrically coupled to the generator 232 to receive electrical power. The evaporator and its associated fan may be positioned in or otherwise in thermal communication with the compartment 226.

By configuring the system (either mechanically or via controller programming or hardwiring) so that one port of each reservoir is always open, the possibility of reservoir overpressure is substantially eliminated. This may allow omission of special means for preventing overpressure (e.g., separate systems for cooling the reservoirs).

Although basic systems have been illustrated, more complex implementations are possible involving further features of either the reservoirs or the basic refrigeration circuit. Additional components, flowpaths, etc., may be present.

One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, when implemented in the retrofit/remanufacture of an existing system or a reengineering of the existing system configuration, details of the existing configuration may influence details of the particular implementation. Accordingly, other embodiments are within the scope of the following claims. 

1. A refrigeration system (20) comprising: a compressor (22); a first heat exchanger (24); an expansion device (26); and a second heat exchanger (28), a refrigerant recirculating flowpath (40) extending sequentially downstream through the compressor, first heat exchanger, expansion device, and second heat exchanger, characterized by: a first refrigerant storage chamber (85); a second refrigerant storage chamber (86), and at least one valve (90, 91, 96, 97) coupled to the first refrigerant storage chamber and second refrigerant storage chamber to permit the first and second refrigerant storage chambers to each be individually placed in alternative communication with the flowpath upstream and downstream of the expansion device.
 2. The system of claim 1 wherein: the second refrigerant storage chamber is larger than the first refrigerant storage chamber.
 3. The system of claim 2 further comprising: a third refrigerant storage chamber (87), larger than the second refrigerant storage chamber.
 4. The system of claim 1 further comprising: a third refrigerant storage chamber (87).
 5. The system of claim 1 wherein: there are no additional refrigerant storage chambers.
 6. The system of claim 1 further comprising: a control system (66) coupled to the at least one valve and configured to: select a charge storage condition from a plurality of pre-determined conditions; and operate the at least one valve to place the system in the selected charge storage condition.
 7. The system of claim 1 further comprising: a transport container (224) having a compartment (226) positioned in thermal communication with the second heat exchanger.
 8. The system of claim 7 further comprising: an internal combustion engine-powered generator (230, 232) coupled to the compressor to power the compressor.
 9. The system of claim 1 wherein: a refrigerant charge of the system is at least 50% carbon dioxide by weight.
 10. A refrigeration system (20) comprising: a compressor (22); a first heat exchanger (24); an expansion device (26); a second heat exchanger (28), a refrigerant recirculating flowpath (40) extending sequentially downstream through the compressor, first heat exchanger, expansion device, and second heat exchanger; and means (90, 91, 92, 96, 97, 98) for selectively diverting refrigerant from the flowpath to a plurality of chambers (85, 86, 87) and returning the refrigerant to the flowpath while maintaining the chambers at pressure below a peak pressure of the flowpath
 11. The system of claim 10 wherein: a refrigerant charge of the system is at least 50% carbon dioxide by weight.
 12. The system of claim 10 further comprising: a transport container (224) having a compartment (226) positioned in thermal communication with the second heat exchanger.
 13. A refrigeration system operating method comprising: compressing a refrigerant; passing the compressed refrigerant through a first heat exchanger (24) downstream of a compressor (22) along a refrigerant flowpath (40); expanding the refrigerant downstream of the first heat exchanger along the refrigerant flowpath; passing the expanded refrigerant through a second heat exchanger (26); returning the refrigerant to the compressor; and diverting refrigerant to a storage unit (80) and returning the refrigerant from the storage system, characterized in that: the storage unit has a plurality of chambers (85, 86, 87); the storage unit includes at least one valve (90, 91, 92, 96, 97, 98) positioned to selectively place each chamber in communication with the flowpath; and the diverting and returning comprises actuating the at least one valve to place each of the chambers in communication with the flowpath either upstream of or downstream of the expansion device.
 14. The method of claim 13 wherein the diverting and returning comprises: determining a desired charge storage condition from a plurality of predetermined conditions; and actuating the at least one valve to achieve the desired charge storage condition.
 15. The method of claim 13 wherein: the compressed refrigerant is passed through the first heat exchanger in a supercritical condition.
 16. The method of claim 13 wherein: the diverting and returning comprises operating with eight different nominal charge storage configurations.
 17. The method of claim 13 wherein: the diverting and returning comprises operating with four to eight different nominal charge storage configurations. 